1. Field of the invention
This invention relates to a toner for developing electrostatic latent images, used in an image forming process such as electrophotography or electrostatic printing, a process cartridge having such a toner and an image forming method making use of the toner.
2. Related Background Art
A number of methods as disclosed in U.S. Pat. No. 2,297,691, Japanese Patent Publications No. 42-23910 and No. 43-24748 and so forth are hitherto known as electrophotography. In general, copies or prints are obtained by forming an electrostatic latent image on a photosensitive member by utilizing a photoconductive material, subsequently developing the latent image by the use of a toner, and transferring the toner image to a transfer medium such as paper if necessary, followed by fixing by the action of heat, pressure, heat-and-pressure, or solvent vapor. The toner not transferred to and remaining on the photosensitive member is cleaned by various means, and then the above process is repeated.
In recent years, such copying apparatus are severely sought to be made small-sized, lightweight and highly reliable. As a result, a high performance has become required for toners. For example, various methods or devices have been developed in relation to the step of fixing a toner image to a sheet such as paper. A method most commonly available at present is the pressure heat system using a heat roller. The pressure heat system using a heat roller is a method of carrying out fixing by causing a toner image surface of an image-receiving sheet to pass the surface of a heat roller whose surface is formed of a material having releasability from toner while the former is brought into contact with the latter under application of a pressure. Since in this method the surface of the heat roller comes into contact with the toner image of the image-receiving sheet under application of a pressure, a very good thermal efficiency can be achieved when the toner image is fixed onto the image-receiving sheet, so that the fixing can be carried out rapidly. Thus, this method is very effective in high-speed electrophotographic copying machines.
The heat-roll fixing having been hitherto widely used, however, has the following problems:
(1) A time during which an image-forming operation is prohibited, i.e., what is called a waiting time, is required until the heat roller reaches a given temperature.
(2) The heat roller must be maintained at an optimum temperature in order to prevent poor fixing from being caused by the variations of the heat-roller temperature that may occur when the recording medium is passed or because of other external factors, and also to prevent the phenomenon of offset of toner to the heat roller. This requires that the heat capacity of the heat roller or a heater element be large, which requires substantial electric power and also causes in-machine temperature rise in the image forming apparatus.
(3) When the recording medium passes over the heat roller, the recording medium and the toner on the recording medium are slowly cooled because of a low temperature of the heat roller, resulting in high adhesion of the toner. Thus, such a state of toner and the curvature of the roller together often causes offset, or paper jam due to the winding of the recording medium around the roller.
However, in addition to the above factors of fixing means, the properties of toner are very important in order to realize a fixing method that may require only a short waiting time and a low consumption of electric power while achieving excellent performance of fixing toner images to a recording medium and excellent anti-offset properties.
For the purpose of causing no toner to adhere to the surface of a fixing roller, it has been hitherto put into practice to add a wax such as low-molecular weight polyethylene or low-molecular weight polypropylene that may well melt at the time of heating, in order to increase the release properties of the toner. This is effective for preventing offset, but on the other hand results in an increase in agglomerating properties of toner to tend to make charging performance unstable and tends to cause a lowering of running performance. Accordingly, as other methods, it has been variously attempted to improve binder resins.
For example, a method is known in which the glass transition temperature (Tg) and molecular weight of a binder resin in toner are increased to improve the melt viscoelasticity of the toner. Such a method, however, raises the problem that the improvement in anti-offset properties may result in a lowering of fixing performance to cause a lowering of the fixing performance in low-temperature fixing, i.e., low-temperature fixing performance, which is required for the achievement of high-speed development and energy saving.
In general, in order to improve the low-temperature fixing performance of toners, it is necessary to decrease the viscosity of toner at the time of its melting, and increase the area in contact with a fixing substrate. For this reason, it is required to lower the Tg and molecular weight of binder resins used.
The low-temperature fixing performance and the anti-offset properties conflict with each other in some phase, and hence it is very difficult to provide toners satisfying these performances at the same time.
To solve this problem, for example, a toner comprising a vinyl polymer cross-linked to an appropriate degree by adding a cross-linking agent and a molecular weight modifier is proposed in Japanese Patent Publication No. 51-23354. In Japanese Patent Publication No. 55-6805, a toner is proposed having as a constituent an .alpha.,.beta.-unsaturated ethylene monomer and made to have a broad molecular weight distribution so that the ratio of weight average molecular weight to number average molecular weight (Mw/Mn) is 3.5 to 4.0. A toner having a blend type resin of a vinyl polymer whose Tg, molecular weight and gel content are specified is also proposed.
The toners according to these proposals certainly have a broader fixing temperature range between the lowest fixing temperature (the lowest temperature at which the fixing is possible) and the offset temperature (the temperature at which the offset begins to occur). There, however, has been the problem that it is difficult to make their fixing temperature sufficiently low when a satisfactory anti-offset performance is imparted to the toner and on the other hand the anti-offset performance becomes low when importance is attached to the low-temperature fixing performance.
Japanese Patent Application Laid-open No. 57-208559 discloses a toner comprising, in place of such vinyl type resins, a polyester resin which is considered substantially superior to the vinyl type resins in view of low-temperature fixing performance, the polyester resin being cross-linked and also being incorporated with an anti-offset agent. This toner is superior in both the low-temperature fixing performance and anti-offset properties, but has a problem in respect of the productivity (grindability) of toner.
Japanese Patent Application Laid-open No. 56-116043 also discloses a toner making use of a resin made polymeric by polymerizing vinyl monomers in the presence of a reactive polyester resin and inserting cross-linking reaction, addition reaction or grafting reaction in the course of the polymerization. This toner has achieved an improvement in respect of grindability, but, in respect of low-temperature fixing performance and anti-offset properties, it is difficult to make full use of the function of each resin.
Japanese Patent Publication No. 1-15063 discloses a toner making use of a resin composition prepared by simply blending a polyester resin with two kinds of vinyl resins having different gel contents (with a degree of gelation of 80% or more and a degree of gelation of less than 10%). This toner is satisfactory in respect of low-temperature fixing performance, but has room for an improvement in respect of anti-offset properties and grindability. If the proportion of the vinyl resin having a gel content of 80% or more is increased for the purpose of improving the anti-offset properties, the anti-offset properties may be improved but on the other hand the low-temperature fixing performance tends to lower. Merely incorporating the vinyl resin having a gel content of less than 10% makes it difficult to satisfy a sufficient grindability when toners are produced.
Meanwhile, with regard to physical properties required in the toners as shown above, it is proposed that a binder resin is cross-linked by allowing a polymer having a carboxylic acid group to react with a metal compound (Japanese Patent Applications Laid-open No. 57-178249 and No. 57-178250), or that a binder having as essential constituents a vinyl resin monomer and a special monoester compound is allowed to react with a polyvalent metal compound to carry out cross-linking through a metal (Japanese Patent Applications Laid-open No. 61-110155 and No. 61-110156).
Japanese Patent Applications Laid-open No. 63-214760, No. 63-217362, No. 63-217363 and No. 63-217364 disclose that a binder resin has a molecular weight distribution separated into two groups, a low-molecular weight region and a high-molecular weight region, and carboxylic acid groups incorporated into the low-molecular weight region side are allowed to react with polyvalent metal ions to carry out cross-linking (a dispersion of a metal compound is added in a solution obtained by solution polymerization, followed by heating to carry out the reaction). In any methods disclosed therein, it is difficult to react well the binder with the metal compound or to disperse well the metal compound in the binder resin. Thus, it is sought to more improve the properties required in toners, in particular, fixing performance and anti-offset properties. Moreover, since the metal compound must be mixed in the binder resin in a large quantity, the metal compound mixed may act as a catalyst on the binder resin under some conditions, and the binder resin tends to undergo gelation. As a result, it is difficult to mix the metal compound and determine production conditions for obtaining the desired toners. Even if the production conditions could be determined, there is the problem of a difficulty in reproducibility.
Adjustment of acid values of these toners leaves room for further improvement in charging performance (charging rise) of toner, environmental properties (high-humidity storage stability) and image characteristics (fog and density).
Japanese Patent Applications Laid-open No. 2-168264, No. 2-235069, No. 5-173363, No. 5-173366 and No. 5-241371 further disclose toner binder resin compositions and toners in which the molecular weights, mixing ratio, acid values and percentages of low-molecular weight components and high-molecular weight components in binder resins are controlled to improve fixing performance, anti-offset properties, image characteristics, anti-blocking performance, charging rise performance and so forth. There, however, is room for further improvement.
In particular, in the adjustment of acid values mentioned above, the dispersibility of colorants such as magnetic iron oxides and the dispersibility of charge control agents (charging controllers) and other additives tend to become poor, to tend to cause fog due to contamination on the surface of a carrier or a developer carrying member such as a developing sleeve, or image deterioration such as a decrease in image density.
For the achievement of much lower-temperature fixing, it is sought to make further improvements.
Japanese Patent Application Laid-open No. 62-9256 discloses a toner binder resin composition comprising a blend of two kinds of vinyl resins having different molecular weights and acid values of resin. However, when such a binder resin is used, kneading conditions must be made strict in order to improve the compatibility and dispersibility of the components constituting the toner. Such a binder resin is liable to undergo a break of polymer molecules, and hence it becomes difficult to exhibit the desired performance on the anti-offset properties. If materials are kneaded to such an extent that the resin may not undergo a break of polymer molecules, the dispersibility of other additives may become poor to accelerate the contamination on the surface of a carrier or a developer carrying member-to tend to cause the problems of fog and black spots around line images. Especially when a polymer with a weight average molecular weight of 1,000,000 or more is used, these phenomena tend to appear.
Japanese Patent Application Laid-open No. 3-72505 discloses a vinyl type toner binder resin with a molecular weight of 300,000 or more, employing a polyfunctional initiator. The use of such a resin enables satisfaction of fixing performance to a certain extent, but, in addition to the above problems, tends to cause a lowering of performance when toner is left to stand at high temperatures. The cause of this phenomenon is unclear, and is presumed to be due to the fact that the break of binder resin molecules is accelerated in the production of toner and hence the proportion of high-molecular weight resin components in the toner composition decreases to cause a lowering of thermal resistance.
The various performances required for toners often conflict with one another, and yet in recent years it is sought to satisfy all of them in a high performance. Moreover, it is sought to take a general measure which includes developing performance.
For the achievement of high-speed processing in electrophotographic apparatus as required in recent years, toners are sought to have more low-temperature fixing performance and also have a toner toughness high enough to endure high-speed development and a charging stability high enough to endure long-term running.
However, with regard to the low-temperature fixing performance and toner toughness as aimed herein, it is difficult to achieve both at the same time.
U.S. Pat. No. 3,909,258 discloses a developing method employing a magnetic toner having an electrical conductivity. This is a method in which a conductive magnetic toner is supported on a cylindrical conductive sleeve internally provided with a magnet, and the toner is brought into contact with electrostatic images to carry out development. In this development, in the developing zone, a conductive path is formed between the surface of a photosensitive member and the surface of the sleeve by toner particles, and charges are led from the sleeve to the toner particles through this conductive path, where the toner particles adhere to image areas by the Coulomb force acting between toner particles and image areas of the electrostatic images. Thus, the electrostatic images are developed. This development carried out using a conductive magnetic toner is a superior method which avoids the problems involved in the conventional two-component development. On the other hand, since the toner is conductive, there is the problem that it is difficult to electrostatically transfer the developed image from the photosensitive member to a final transfer medium such as paper.
As a developing method using a high-resistivity magnetic toner that enables electrostatic transfer, there is a developing method utilizing dielectric polarization of toner particles. Such a method, however, has problems in that the development speed is substantially low and the density of developed images is not well attained.
As other methods using a high-resistivity insulating magnetic toner, methods are known in which toner particles are triboelectrically charged by the mutual friction between toner particles, by the friction between toner particles and a developing sleeve or by the friction between toner particles and a blade or a coating roller, and the toner particles thus charged are moved to an electrostatic image bearing member. Such methods, however, have had the problems that the triboelectric charging tends to become insufficient because of a small number of contact times between the toner particles and the friction member and the toner particles charged tend to agglomerate on the sleeve because of the Coulomb force increased between the toner particles and the sleeve.
Japanese Patent Application Laid-open No. 55-18656 discloses a novel jumping development that has eliminated the above problems. This is a method in which a magnetic toner is very thinly applied to a developing sleeve, and the toner thus applied is triboelectrically charged, which is then conducted very close to electrostatic images to carry out development. According to this method, since the magnetic toner is very thinly applied to the developing sleeve, the opportunities of contact between the developing sleeve and the toner increase to enable sufficient triboelectric charging, and also since the magnetic toner is supported by magnetic force and the magnet and the toner are relatively moved, the toner particles are released from their mutual agglomeration and can be sufficiently brought into friction with the sleeve, whereby superior toner images can be obtained.
However, a finely divided magnetic material is mixed and dispersed in the insulating magnetic toner in a considerable quantity and the magnetic material is partly laid bare to the surfaces of toner particles, and hence the properties of the magnetic material affect the fluidity and triboelectric chargeability of the magnetic toner, consequently tending to cause variation or deterioration of various performances such as developing performance and running performance required in magnetic toners.
In the jumping development making use of a magnetic toner, as a result of continual repetition of a developing step (e.g., copying) over a long period of time, the fluidity of the developer containing the magnetic toner may lower to make it difficult to achieve normal triboelectric charging, so that the charging tends to become non-uniform, and fogging tends to occur in an environment of low temperature and low humidity, tending to cause problems on toner images. In the case where the binder resin and magnetic material constituting magnetic toner particles have a weak adhesion, the magnetic material may come off the surfaces of toner particles as a result of the repeated developing step to tend to adversely affect the toner images, e.g., to cause a decrease in image density.
In the case when the magnetic material is not uniformly dispersed in the magnetic toner particles, magnetic toner particles containing the magnetic material in a large quantity and having small particle diameters may accumulate on the developing sleeve to cause a decrease in image density and an uneven light and shade called sleeve ghost in some instances.
Proposals concerning magnetic iron oxides to be contained in magnetic toners are hitherto made, but there is room for further improvement.
For example, Japanese Patent Applications Laid-open No. 62-279352 and 62-278131 disclose a magnetic toner containing a magnetic iron oxide incorporated with silicon element. In such a magnetic iron oxide, the silicon element is intentionally added inside the magnetic iron oxide, but there is room for further improvement in the fluidity of the magnetic toner containing the magnetic iron oxide.
Japanese Patent Publication No. 3-9045 discloses adding a silicate to control the shape of magnetic iron oxide to be spherical. In the magnetic iron oxide thereby obtained, the silicon element is richly distributed inside the magnetic iron oxide because of the use of the silicate for controlling particle diameter and the silicon element is less present on the surface of the magnetic iron oxide, so that the improvement in fluidity of the magnetic toner tends to become insufficient.
Japanese Patent Application Laid-open No. 61-34070 discloses a process for producing triiron tetraoxide by adding a hydroxosilicate solution to triiron tetraoxide in the course of oxidation reaction. The triiron tetraoxide obtained by this process has silicon element in the vicinity of its surface, but the silicon element is present in layer in the vicinity of the surface of the triiron tetraoxide. Hence, there is the problem that the surface is weak to mechanical shock such as friction.
Japanese Patent Application Laid-open No. 5-72801 discloses a magnetic toner containing magnetic iron oxide incorporated with silicon element and in which 44 to 84% of silicon element of the whole silicon content is present in the vicinity of the surface of the magnetic iron oxide.
The magnetic toner containing such magnetic iron oxide has brought about improvements in the fluidity of toner and the adhesion to binder resin. However, because of the local presence of silicon element in the vicinity of the surface of the magnetic iron oxide particles, such a toner tends to cause a lowering of environmental properties, in particular, a lowering of charging performance when left standing for a long period of time in an environment of high humidity.
Japanese Patent Application Laid-open No. 4-362954 also discloses a magnetic iron oxide containing both silicon element and aluminum element. It, however, is sought to more improve environmental resistance.
Japanese Patent Application Laid-open No. 5-213620 still also discloses a magnetic iron oxide containing a silicon component and in which the silicon component is laid bare to the surface. It, however, is sought to more improve environmental resistance.
Moreover, the above magnetic iron oxide containing silicon element which is largely present in the vicinity of the surface of the magnetic material tends not to be well dispersed in the binder resin. In order to make full use of the excellent properties possessed by the respective constituents of toners, it is required in the designing of toners to select materials having good compatibility and physical mixing properties with the respective components.